14 



SILVA OF NORTH AMERICA. 



CONIFERJE. 



Leaves in 2 or 3-leaved clusters. 



Leaves in 3 or in 2 and 3-leaved clusters, from 3 to 15 inches in length ; cones from 

 3 to 12 inches long, in falling separating from the lower scales persistent on the 

 peduncle 16. P. ponderosa. 



Leaves in 3-leaved clusters, slender, pale green, from 2h to 4 inches in length, their 

 sheaths deciduous ; cones broadly ovate, from l£ to 2 inches long, maturing at the 

 end of the third season, their scales slightly thickened, furnished with small recurved 

 deciduous prickles 17. P. Chihuahuana. 



Leaves in 2-leaved clusters, closely serrulate, from 1 to 4 inches long ; cones oblong- 

 oval, oblique, more or less serotinous, their scales often tuberculate, and armed with 



slender prickles 18. P. contorta. 



Taedae. Cones lateral, their scales much thickened, variously armed ; leaves in 2 or in 3-leaved 

 clusters, their sheaths persistent ; fibro-vascular bundles 2 ; resin ducts parenchymatous. 

 Leaves in 3-leaved clusters. 



Leaves slender, drooping, pale blue-green, from 8 to 12 inches in length ; cones oval, 

 acute, from 6 to 10 inches long, their scales produced into prominent umbos armed 

 with stout straight or slightly incurved spines 19. P. Sabiniana. 



Leaves stout, erect, dark blue-green, from 6 to 12 inches in length ; cones elongated- 

 oval, acute, from 10 to 14 inches long, their scales much thickened into stout elon- 

 gated umbos armed with thick spur-like incurved spines 20. P. Cotjlteri. 



Leaves slender, bright green, from 4 to 6 inches in length ; cones oval, oblique, 

 from 3 to 6 inches long, persistent, their scales mammillate on the outer side, armed 

 with minute incurved prickles 21. P. RADIATA. 



Leaves pale yellow-green, from 5 to 7 inches in length ; cones elongated-conical, 

 oblique, clustered, from 3 to 5 inches long, serotinous, their scales unequally embossed, 

 armed with stout prickles 22. P. Attenuata. 



Leaves slender, pale green, from 6 to 9 inches in length ; cones ovate-oblong, from 3 



to 5 inches long, their scales armed with stout recurved prickles 23. P. T.a)DA. 



Leaves stout, rigid, dark yellow-green, from 3 to 5 inches in length ; cones ovoid- 

 conical or ovate, often clustered, from 1 to 3^ inches long, their scales armed with 

 short stout recurved prickles 24. P. RIGIDA. 



Leaves slender, dark yellow-green, from 6 to 8 inches in length ; cones usually sub- 

 globose, or elongated, from 1\ to 3 inches long, serotinous, their scales armed with 



slender incurved deciduous prickles 25. P. SEROTINA. 



Leaves in 2-leaved clusters, except in No. 31. 



Leaves stout, gray-green, from \\ to 3 inches in length; cones oblong-conical, often 

 more or less curved, from 2 to 3 inches long, armed with slender straight or in- 

 curved prickles 26. P. Virginiana. 



Leaves slender, flexible, dark green, from 2 to 3^ inches in length ; cones ovoid- 

 conical, serotinous, persistent for many years, their scales armed with short stout 

 straight or recurved spines 27. P. clausa. 



Leaves soft, slender, dark green, from 1 1 to 3 inches in length ; cones subglobose 

 to oblong-ovate, from 1^ to 2 inches long, their scales thin, tipped with straight 

 or recurved short often deciduous prickles 28. P. glabra. 



Leaves stout, blue-green, from 1^ to 1\ inches in length ; cones oblong-conical, oblique, 



from 2 to 2>\ inches long, their scales armed with stout hooked spines 29. P. pungens. 



Leaves rigid, dark green, from 4 to 6 inches in length ; cones ovate, oblique, serotinous, 



persistent, from 2 to 3^ inches long, their scales armed with stout incurved spines 30. P. mttricata. 



Leaves slender, dark blue-green, in 2 or in 3-leaved clusters, from 3 to 5 inches in 

 length ; cones ovate or oblong-conical, from 1^ to 2\ inches long, their scales armed 

 with minute slender prickles 31. P. echinata. 



Leaves stout, falcate, divergent, dark gray-green, from f to 1^ inch in length ; cones 

 oblong-conical, oblique, usually erect, incurved, from 1^ to 2 inches long, their scales 



furnished with minute incurved often deciduous prickles 32. P. DIVARIGATA. 



Resin ducts of the leaves internal. 



Australes. Cones subterminal or lateral, their scales conspicuously umbonate ; leaves in 3, or 

 in 2 and 3-leaved clusters ; fibro-vascular bundles 2. 



